In certain electrostatographic imaging and recording processes, for instance, in electrophotographic copying processes, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder which is thereafter fused to a substrate. The fusion step commonly consists of passing the substrate, such as a sheet of paper on which toner powder is distributed in an imagewise pattern, through the nip of a pair of rolls. At least one of the rolls is heated and has a resilient surface. A persistent problem in this operation is that when the toner is heated during passage through the rolls it may tend to adhere not only to the paper but also to the fusing roll which contacts it. Any toner remaining adhered to the roll can cause a false offset image to appear on the next sheet that passes through the rolls and can also degrade the fusing performance of the roll.
To prevent toner offset many expedients have been tried such as covering the rolls with fluorocarbon polymers or silicone polymers of low surface energy. Also, poly(dimethylsiloxane) (also referred to as PDMS) oils have been applied as release liquids to the roll surfaces. With such materials, however, problems can occur.
One problem is that fluorocarbon polymers are difficult to wet with PDMS release oils, and the application of excessive amounts of such oils to roll surfaces in order to achieve sufficient roll-surface wetting, can cause oil stains on the paper to which toner is being fused.
A major problem is the effect that the PDMS release liquids can have on the fusing roll. Although PDMS oils aid in preventing toner build-up on the rolls, they cause another problem, because they are compatible with poly(dimethylsiloxane) rubbers that are widely used as fusing roll covers. The poly(dimethylsiloxane) oils are absorbed by the poly(dimethylsiloxane) covering of the rolls upon repeated use and cause swelling of the rolls.
Because of the swelling of the rolls, certain defects appear in thermally fixed images. In particular, "step patterns" appear in the images when using various copy sheet sizes. These result from the differential swelling of the fuser roll inside and outside of the paper with, which causes nonuniform roll compression when different sizes of copy paper are used. There can also be increased wear on the roll and shortened useful fusing roll life, because of softening of the roll surface and degrading interaction of PDMS oil with the core or with adhesive interlayers.
Another fault of poly(dimethylsiloxane) rubber polymers is that they provide fuser roll covers having a lower than desirable degree of thermal conductivity, which leads to inefficient heating of the fuser roll and inefficient heating of the toner to be fused. Also, if the fuser roll is internally heated, inefficiencies in heating can require use of high heating temperatures that can contribute to shorter fuser roll life by causing thermal degradation, especially at the interface of the fuser roll core and cover.
U.S. Pat. No. 4,430,406, discloses that fusing roll swelling can be controlled by spraying a fluorocarbon elastomer overcoat on the silicone elastomer roll cover. This method is costly, however, and only partially solves the problem.
U.S. Pat. No. 4,515,884 discloses the use of a release oil having a viscosity in the range from 7,000 to 20,000 centistokes, which allegedly reduces the problem. Even these materials, however, can lead to step patterns in the images.
Because of the swelling problems encountered with poly(dimethylsiloxane) fusing roll covers, it has been suggested in Japanese Kokai No. 59-209129, published Nov. 27, 1984, to make the roll covering from polymers containing some methylphenylsiloxane repeating units. Such polymers would perhaps be less compatible with poly(dimethylsiloxane) oils and hence less subject to swelling. Unfortunately, such polymers can degrade during preparation or use to yield siloxane compounds having single phenyl groups attached to silicon, which have been shown to exhibit estrogenic and sterility effects, making them undesirable for human contact.
It would be desirable to be able to fashion a fusing roll having an outer covering that provides all the benefits of poly(dimethylsiloxane) roll coverings and is also more thermally conductive, is more resistant to swelling by PDMS release oils, is thermally stable, and cannot yield estrogenic degradation products. The present invention provides such a fusing roll and a method for making it.